1. Field of the Invention
A housing for a capacitive pressure sensor for use in the measurement of pressure in a fluid medium, which medium may be either liquid or gas.
2. Prior Art
Capacitive pressure sensors as known in the prior art generally teach a pressure responsive diaphragm forming one plate or electrode of a capacitor. These sensors, including their electrodes or plates, are generally seated or housed within an enclosure. This enclosure secures the sensors and provides a protective environment for the relatively sensitive electronics and their connecting leads. A protective environment is necessitated by the harsh circumstances in which many of these devices are utilized. An example of such an environment is that of an automobile engine where the pressure sensors are utilized to measure oil pressure, gas pressure, fuel line pressure, etc. An independent housing or protective means for a pressure sensor may require the marriage of the attributes of two dissimilar materials for a particular application to utilize their inherent physical properties. Such usage of incompatible materials provides a means for ease of manufacture, calibration, and improved economies.
There are inherent problems in the capacitive structures or capacitive sensor structures taught in today's technology. Included among these are the use of ceramics for capacitive sensors which ceramics are generally hard and brittle. While useful in many applications, a very hard material provides problems for a broader range of applications. Techniques are continually being developed to mask or overcome such problems while continuing to utilize the relatively low cost ceramic material, which has a thermal or electrical insulating characteristic and relatively low flexural and tensile strength. These insulating characteristics of the ceramic material are desired in capacitive sensors. However, the ceramic must be protected to compensate for its lack of flexural strength and to protect the electronics associated with such capacitive sensors when utilized in a hostile environment. Housings for capacitive sensors require ease of assembly to the ceramic, fatigue strength, tensile strength, high temperature tolerance, and ease of manufacture. Further, these materials must lend themselves to very small sizes as a primary concern of any such sensor is its relative size.
A recognition of many of the problems associated with sensing means and their housings are discussed throughout the art. U.S. Pat. No. 3,859,575 (Lee et al.) discusses some of the physical problems which must be overcome, such as relief of stress upon the structure and the thermal affects on the assembly in an inhospitable environment. In this patent they have taught the use of stress relieving to eliminate hysteresis from the sensing connecting structure and further, to utilize such structure to decrease the thermal path lengths to reduce thermal affects upon such sensors. The electrically conductive surfaces are insulated from the central connecting means and each other in this patent. This disclosure recognizes the physical parameters needed for such applications, that is, the utilization of a fabricated hardened steel to provide good elastic qualities and low hysteresis. Such sections may also be of stainless steel. A glass may be utilized as a suitable insulating material to provide extremely small thermal shifts. It's questionable what is referred to as a glass although quartz may be implied, and it is clearly recognized that the insulating characteristics required for such applications are inherent in any electrically conductive application.
U.S. Pat. No. 4,089,036 (Geronime) teaches a capacitive-type load cell utilizing a support, a diaphragm member mounted with respect to such support and a conductive surface with a capacitor plate attached thereto. The diaphragm must have free edge bending characteristics at its outer peripheral edges as well as between the diaphragm and a load support button to reduce radial bending stresses during loading. Again, the physical characteristics required by such load cells or transducer assemblies is perceived. The recognition of the difficulty of having a metal object as a housing member is noted and requires the use of that housing as a conductive surface. The use of a housing structure as disclosed in Geronime '036 is cognizant of the effect of thermal transients acting upon such a load cell. The structure and orientation of the capacitor plates along with the associated surfaces of the housing attempt to overcome the effect of the stress loading and the thermal transients associated with these stresses. Although load cells are frequently associated with large masses, the harsh environmental characteristics frequently associated with these applications lend themselves the use of such transducer or capacitive sensor assemblies.
U.S. Pat. No. 4,125,027 (Clark) takes note of the characteristics which are preferred in the application of a gage utilizing a load cell-type structure for pressure measurement. In this disclosure, a tubular portion is utilized as a housing. This tubular portion is shown at lower portion 12 in FIGS. 1 through 3. Incorporated within such lower portion is a stator or stator assembly of a ceramic material whereas the housing 12 is of a conductive material, which would include most metallic elements and alloys. Further, this gage is taught to be adaptable for temperature compensation.
A force responsive transducer is taught in U.S. Pat. No. 4,295,376 (Bell) wherein a capacitive-type sensor is housed in a generally rectangular structure to accommodate the electronic circuitry associated with the transducer. This capacitive transducer recognizes the rigidity of glass and ceramic elements. Further, it recognizes that torsional and bending moments within such transducers must be accounted for. Bell '376 takes note of the fact that the enclosure should be sealed to provide a gas-type relationship to the diaphragm cavity or body especially where gas leakage must be avoided.
U.S. Pat. No. 4,382,385 (Paros) teaches a differential pressure transducer utilizing a beam and pivot arrangement. The beam is operable by a single or dual bellows arrangement within a housing, however, the assembly of the patent to Paros '385 takes note of the pressures and their affects upon such constructions. To this end the first and second bellows isolate the resonator from the pressure inputs and a reducer means is provided to reduce the pressure error to an acceptable level by reducing the effective area of the first bellows relative to the area of the pressure sensing diaphragm. This arrangement takes note of the fact that such structures are stress sensitive.
The U.S. Pat. No. 4,382,377 (Kleinschmidt et al.) teaches a pressure sensor for an internal combustion engine utilizing a piezoceramic transducer connected in an operating fashion to the area to be sensed by way of a plunger having a membrane at its end and the sensor is secured in a cylinder head with the membrane located within such cylinder. The piezoceramic device is secured in a housing 17 and the pressure signal is transmitted through plunger 26 and tubular section 16. Kleinschmidt '377 teaches a prestressed ceramic device in housing 17. However, housing 17 is not taught to be of a specific material although it would appear to be of a material to compensate for the heat as well as being machinable as to be threaded. The threaded connection is connected to a cylinder head in a gas-tight manner with a wrench. This disclosure or patent takes note of the need for guaranteeing a longer service life of a thermally stressed, highly loaded membrane 15 no longer under a continuous mechanical prestress. It also recognizes the need for additional protection of the ceramic plate 21 of transducer 20 against high temperatures in a hostile environment in this case from a cylinder of an automobile engine.